Healthy sleep of good quantity and quality is increasingly viewed as a protective factor against the development and/or exacerbation of pain. However, deficient sleep, such as insomnia, is increasingly common in the general population and is highly co-morbid with chronic pain conditions. Though there is an established relationship between the presence of insomnia and the onset or worsening of pain symptoms, the mechanisms underlying this relationship are unknown. The goal of this proposal is to investigate two promising mechanistic candidates underlying the insomia-to-pain directionality: (1) Inflammation: Pro-inflammatory markers (interleukin-6, prostaglandin E2) increase and anti- inflammatory markers (resolvins) decrease when sleep is deficient, and contribute to the sensitization of the nociceptive system (i.e., increased responsiveness to pain signals). (2) Pain inhibition: The capacity to inhibit pain, a central process to control incoming pain signals, is deteriorated when sleep is deficient, thus preventing habituation to pain (decreased responsivness to pain signals). These two mechanistic candidates will be tested in a novel model of repeated exposure to experimentally-induced insomnia, characterized by (1) induction of typical insomnia symptoms (delayed sleep onset, sleep disruption, early morning awakening) and (2) repeated induction of such simulated insomnia episodes, which allows for the investigation of a key feature of many biological systems, i.e. the ability of systems to adapt to repeated stressor or challenge, such as pain. We hypothezise that repeated exposure to insomnia increases vulnerability to chronic pain by promoting two processes: (1) sensitization of the nociceptive system via a progressive increase of the inflammatory response and (2) decreased habituation to pain via a progressive deterioration of the pain-inhibitory response. Twenty-six healthy women and men will undergo two intra-individual balanced 17- day in-hospital stays (insomnia symptoms induction vs control sleep condition), in which frequent blood and urine sampling and a complex pain testing battery will be utilized to investigate the following aims: Aim 1 postulates a progressive increase of the inflammatory response (IL-6, PGE2, resolvins) to repeated exposure to experimentally-induced insomnia symptoms, leading to sensitization of the nociceptive system as manifested by lower pain thresholds to pressure and heat, as well as increased temporal summation of pain (an index of central sensitization). Aim 2 postulates a progressive deterioration of the pain-inhibitory response (measured by the conditioned pain modulation test), contributing to less habituation to pain. Aim 3 postulates a progressive impairment of the ability of the inflammatory and pain-inhibitory system to return to baseline upon repeated exposure to experimentally-induced insomnia symptoms. This research is fundamental for the future development of novel strategies targeting specific mechanisms to prevent or reduce pain exacerbated by insomnia.